There has been known a semiconductor switch element used as a switching element adapted in use for switching ON and OFF a power supply. Examples of the semiconductor switch elements include such as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and IGBT (Insulated Gate Bipolar Transistor).
As a driver for driving the MOSFET or IGBT, there has been proposed to use an insulated type DC-DC converter with a transformer.
For example, JP2003-69406A discloses an example of a driver for semiconductor switch element (see FIG. 17). In JP2003-69406A, a high voltage semiconductor switch is realized by a plurality of switching elements Q101 connected in series with each other, where each of the switching elements Q101 is constituted by IGBT. In the configuration of JP2003-69406A, diodes D101 are provided in inverse-parallel connections with the respective switching elements Q101.
In the light of maintaining the DC (direct-current) voltage balance, each of the switching elements Q101 is connected to a corresponding parallel resistor R101. In the light of maintaining the voltage balance when the switch element is turned-off, each of the switching elements Q101 is connected to a corresponding snubber circuit constituted by a diode D103, a resistor R102, and a capacitor C101.
In this configuration, each of the switching elements Q101 is driven by a secondary winding N102 of a corresponding pulse transformer Tr101, where primary windings N101 of the pulse transformers Tr101 are connected in series with each other. With respect to each switching element Q101, a full-wave bridge rectifier DB101, a smoothing capacitor C102, a resistor R103, a diode D102, and a PNP transistor Q102 are connected between the secondary winding N102 of the pulse transformer Tr101 and a gate of the switching element Q101.
In addition, a two-terminal trigger device K101 and a series resistor R104 are connected between the bridge rectifier DB101 and the switching element Q101. A collector of the switching element Q101 is connected to a resonator circuit 101. When an electric voltage larger than a predetermined voltage is applied on the collector of the switching element Q101, the two-terminal trigger device K101 is applied with a breakdown voltage and becomes conductive, and thereby an electric voltage is applied on the gate of the switching element Q101.
In this configuration, the primary windings N101 of the pulse transformers Tr101 are connected in series with a secondary winding N202 of a push-pull transformer Tr102. A push-pull inverter is realized by: a DC control source E101; a primary winding N201 with a center-tap of the push-pull transformer Tr102; and two switching elements Q103, Q104 each of which is formed of a FET and which are controlled to be turned on alternately. A high-frequency pulse generator 102 is constituted by a pulse generator that generates signals with opposite phases so as to alternately turn on the switching devices Q103, Q104.
For the purpose of improving the voltage tightness and reducing an on-resistance of a switching element, there has been studied such a switching element that is formed of a wide-gap semiconductor having a large bandgap. Examples of the wide-gap semiconductors include such as SiC (silicon carbide) and GaN (gallium nitride). For example, the wide-gap semiconductor is defined as a semiconductor having a bandgap twice or more of a bandgap (1.1 [eV]) of silicon (Si) (i.e. semiconductor having a bandgap more than 2.2 [eV]).
As a semiconductor switch FET (Field-Effect Transistor) formed of GaN (which is a kind of wide-gap semiconductor), it has been reported such a FET that has a p-n junction or a schottky junction at its gate and that has a normally-off property. Such a junction-gate type FET has a diode structure in a region between the gate and the source, and an application of an electric voltage induces a gate current. Therefore, in case where the FET is driven by the conventional driver having the circuit configuration shown in FIG. 17, there is a possibility that the diode D102 cannot reach an inversely-biased state and the FET cannot turn-off in a desired speed, due to the gate current.